Silver selenide sputtered films and method and apparatus for controlling defect formation in silver selenide sputtered films

ABSTRACT

Method and apparatus for sputter depositing silver selenide and controlling defect formation in and on a sputter deposited silver selenide film are provided. A method of forming deposited silver selenide comprising both alpha and beta phases is further provided. The methods include depositing silver selenide using sputter powers of less than about 200 W, using sputter power densities of less than about 1 W/cm 2 , using sputter pressures of less than about 40 mTorr and preferably less than about 10 mTorr, using sputter gasses with molecular weight greater than that of neon, using cooling apparatus having a coolant flow rate at least greater than 2.5 gallons per minute and a coolant temperature less than about 25° C., using a magnetron sputtering system having a magnetron placed a sufficient distance from a silver selenide sputter target so as to maintain a sputter target temperature of less than about 350° C. and preferably below about 250° C. during sputter deposition, and heating the sputter deposition substrate to greater than about 30° C.

This application is a continuation of U.S. application Ser. No.12/841,035, filed Jul. 21, 2010, now abandoned, which is a divisional ofU.S. application Ser. No. 10/712,106, filed Nov. 24, 2003, now U.S. Pat.No. 7,799,180, issued Sep. 21, 2010, both of which are incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates generally to a method and apparatus forsputtering silver selenide and more particularly to a method andapparatus for controlling defect formation and structural phases insputtered silver selenide layers.

BACKGROUND OF THE INVENTION

Noble metal doped chalcogenide glasses, such as silver selenide, arepresently of great interest for use in non-volatile memory devices.Non-volatile memory devices employing thin films of doped chalcogenideglasses offer several advantages over conventional devices, especiallywith regard to non-volatile memory speed, thermal characteristics,durability and reliability.

Thin film doped deposition is an important aspect of fabricating memorydevices containing metal doped chalcogenide glasses. Because these filmsplay an important role in the electrical performance of such memorydevices, it is desirable that these deposited layers be defect free.This is especially true when such memory devices are used in ultra largescale integration (ULSI) devices, whose sub-micron structure greatlyincreases the need for thin film defect control. For example, thin filmdefects may be 10 to 100 times larger than typical ULSI devicestructures and thus can easily cause shorting and other electricalperformance problems.

Silver selenide is a desirable chalcogenide glass material for use infabricating non-volatile memory devices. To date, however, there hasbeen only limited research into the fundamental properties of silverselenide thin films. Furthermore, most of this research involves formingsilver selenide films via an evaporation deposition technique.Evaporation deposition, however, is not well suited for industrialapplication and has an attendant problem in that the dissociativeproperties of silver selenide make it difficult to achieve precisionstoichiometries in the deposited film.

Physical vapor deposition, also known as “sputtering,” is more easilyadaptable to industrial applications and also provides better coatingthickness and quality control than evaporation deposition techniques.Silver selenide, however, exhibits defect formation during conventionalsputter deposition due to localized high temperatures, which occurduring the sputtering operation.

Sputtering devices have long been used by the semiconductor processingindustry to coat substrates (e.g., silicon wafers) with variousmaterials (e.g., aluminum, titanium, gold, etc.) during the manufactureof integrated circuits. Generally, in a sputtering device, the materialto be deposited or sputtered onto the substrate is contained in atarget. The substrate is placed on a substrate support table in asputtering chamber. Air in the sputtering chamber is evacuated andreplaced with an inert gas such as argon, usually at a low pressure. Anelectric field is then established between an anode, such as the wallsthat line the sputtering chamber, and the target, which acts as ancathode (electron source). The resulting potential gradient causeselectrons to be emitted from the target surface. As these electrons aredrawn toward the anode by the electric field, they strike and ionizesome of the inert gas molecules. These positively charged inert gas ionsare then drawn toward and collide with the negatively charged target.The ions impact the target with sufficient energy to dislodge, orsputter, particles of target material into the sputtering chamber. Thesubstrate to be coated, which is usually positioned in the chamber withits surface facing the target, receives some of the sputtered targetparticles, which adhere to and coat the substrate. The cloud of freeelectrons, inert gas atoms, inert gas ions, and sputtered targetparticles that exists near the target sputtering surface is termed a“plasma discharge.”

The location of the plasma discharge may be controlled using magnetronsputtering devices, which introduce a magnetic field adjacent to thesputtering surface of the target in the sputtering chamber. The magneticfield is generated by a rotating magnetic circuit located on the side ofthe target opposite the sputtering surface. The magnetic field acts totrap electrons in a desired region, thus producing a region ofhigh-density plasma. This region of high-density plasma rotates with themagnetic circuit about an axis that is perpendicular to the targetsputtering surface. Thus, deep erosion of the target sputtering surfaceoccurs in the region where the high density plasma is produced, whileother portions of the target are hardly sputtered at all. Thispreferentially sputtered region of the target is often referred to asthe racetrack portion, due to the characteristic ovoid-shaped area ofthe target that is eroded by the high-density plasma discharge.

It is generally known that the sputtering process generates asubstantial amount of energy, which results in heating of the sputteringtarget. This heating is caused by the high electrical potential andcurrent applied to the target material and by the energy delivered tothe target by the bombarding ions. The heat generated during sputteringneeds to be dissipated; otherwise it may damage the target and othercomponents of the magnetron sputtering device and, in the case of silverselenide targets, may cause defect formation on the target material. Inone previously known approach for cooling a sputtering target, awater-tight cooling chamber is formed on the side of the target oppositethe target sputtering surface. The non-sputtering surface of the targetforms one wall of the cooling chamber. The cooling chamber is filledwith coolant (e.g., water), which floods the non-sputtering surface ofthe target and dissipates the heat generated during sputtering.

During the conventional sputter deposition of silver selenide using amagnetron sputtering device, millimeter sized defects rapidly form onthe surface of the target, especially in the target racetrack area.Silver selenide target surface defect formation causes increased defectformation in and on the deposited film. This defect formation resultsfrom localized heating of the target during sputtering coupled with boththe relatively low phase transition point of selenide and the lowmelting point of silver. It is thought that the associated heating ofthe Ag₂Se target during sputtering promotes the diffusion of silver intothe Ag₂Se target, which causes natural protrusions to grow on thetarget. These defects are then transferred to the deposited film duringsputtering, causing undesired localized stoichiometric variations in thedeposited film.

Thus, in view of the foregoing, there is a desire and need for animproved method of sputtering a silver selenide target with a reducedtarget temperature, which in turn reduces target and deposited filmdefect formation.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention provide a method and apparatus for sputterdepositing a silver selenide film on a substrate, while maintaining asputter target temperature at least below about 350° C. and preferablybelow about 250° C. Desirable sputtering processes include magnetron RF,DC, and pulse DC sputtering.

In one embodiment, silver selenide films are sputter deposited byemploying a silver selenide target having a temperature maintained belowabout 350° C., more preferably below about 250° C., wherein said targettemperature is maintained by conducting the sputtering process under asputter power below about 200 W, more preferably below about 100 W.

In another embodiment, silver selenide films are sputter deposited byemploying a silver selenide target having a temperature maintained belowabout 350° C., more preferably below about 250° C., wherein said targettemperature is maintained by conducting the sputtering process with asputter gas maintained at a pressure of less than about 40 mTorr, morepreferably less than about 10 mTorr.

In another embodiment, silver selenide films are sputter deposited byemploying a silver selenide target having a temperature maintained belowabout 350° C., more preferably below about 250° C., wherein said targettemperature is maintained by conducting the sputtering process employinga sputter gas with a molecular weight greater than that of neon.

In yet another embodiment, silver selenide films are sputter depositedby employing a silver selenide target having a temperature maintainedbelow about 350° C., more preferably below about 250° C., wherein saidtarget temperature is maintained by conducting the sputtering processusing a target cooling apparatus with coolant flow rates at least above2.5 gal/min and coolant temperature at least below 25° C.

In another embodiment, silver selenide films are sputter deposited byemploying a silver selenide target having a temperature maintained belowabout 350° C., more preferably below about 250° C., wherein said targettemperature is maintained by conducting a sputter process under asputter power density of less than about 1 W/cm².

In another embodiment, silver selenide films are sputter deposited bypositioning a magnetron of the sputtering system such that the silverselenide target temperature is maintained below about 350° C., morepreferably below about 250° C.

Desirable sputter apparatuses of the present invention includesputtering systems with cooling apparatuses capable of maintaining asilver selenide target at a temperature at least below about 350° C. andpreferably below about 250° C. during sputter deposition. Other desiredsputtering apparatuses include magnetron sputtering systems using amagnetron positioned so as to maintain a target temperature of less thanabout 350° C. and preferably less than about 250° C. during sputterdeposition.

The methods and apparatuses of the present invention are particularlyuseful for controlling defect formation during sputter deposition ofsilver selenide thin films.

BRIEF DESCRIPTION OF THE DRAWINGS

The features mentioned above and other features and advantages of thepresent invention will be better understood from the following detaileddescription, which is provided in connection with the accompanyingdrawings in which:

FIG. 1 is a graph showing total particle count vs. target lifetime for asilver selenide sputtering target;

FIGS. 2( a) and 2(b) illustrate scanning electron microscope imagesshowing typical defects deposited on and in a sputter deposited silverselenide film;

FIGS. 3( a) and 3(b) show images of typical black porous defects formedon an Ag₂Se sputter target after 3 kwh of target consumption duringsputter deposition;

FIG. 4( a) shows a deposited silver selenide film formed according toone embodiment of the prior art;

FIG. 4( b) shows a silver selenide film structure change when the silverselenide film of FIG. 4( a) is annealed at 130° C.;

FIG. 4( c) shows further silver selenide film structure changes when thesilver selenide film of FIG. 4( a) is annealed at 250° C.;

FIG. 4( d) shows cone defect formation when the silver selenide film ofFIG. 4( a) is annealed at 350° C.;

FIG. 4( e) shows whisker-like defect formation when the silver selenidefilm of FIG. 4( a) is annealed at 380° C.;

FIG. 4( f) shows millimeter-sized whisker-like defect formation when thesilver selenide film of FIG. 4( a) is annealed at 410° C.;

FIG. 5 is a graph illustrating the relationship between targettemperature and sputter power during silver selenide deposition;

FIG. 6 is a graph illustrating the relationship between targettemperature and sputter pressure during silver selenide sputterdeposition;

FIG. 7 is a graph illustrating the relationship between targettemperature and Ne/Ar sputter gas flow ratio at varying pressures duringsilver selenide sputter deposition;

FIG. 8 is a graph illustrating the relationship between targettemperature and Xe/Ar flow ratio at varying pressures during silverselenide sputter deposition;

FIG. 9 is a graph illustrating the relationship between targettemperature and sputter gas;

FIG. 10 is a graph illustrating the relationship between targettemperature and effective sputter power density;

FIG. 11 shows a simplified example of a magnetron sputtering device tobe used to carry out the embodiments of the invention;

FIG. 12( a) shows a cross sectional view of a non-uniform target sputterprofile;

FIG. 12( b) shows a top view of a non-uniform target sputter profile,which forms a characteristic racetrack shape; and

FIG. 12( c) shows a cross sectional view of a uniform target sputterprofile.

FIG. 13 shows two x-ray diffraction scans of deposited silver selenidefilms sputtered under different sputter pressures and powers.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to variousspecific structural and process embodiments of the invention. Theseembodiments are described with sufficient detail to enable those skilledin the art to practice the invention. It is to be understood that otherembodiments may be employed, and that various structural, logical andelectrical changes may be made without departing from the spirit orscope of the invention.

The term “silver selenide” is intended to include various species ofsilver selenide, including some species which have a slight excess ordeficit of silver, for instance, Ag₂Se, Ag_(2+x)Se, and Ag_(2−x)Se.

The term “defect free silver selenide film” as used in this applicationis defined as silver selenide films having less than about 0.16 defectcounts/cm².

The present invention relates to a method of controlling defectformation in deposited silver selenide and to a method of depositing asilver selenide film having a mixture of alpha and beta phases. Inaccordance with the invention, silver selenide is deposited using asputtering process that maintains the silver selenide target temperatureat less than about 350° C. and preferably less than about 250° C.

During the conventional sputter deposition of silver selenide, targetdefect count has been observed to increase from less than fifty to overthousands per wafer after only very short target consumption (2-3 kwh),as illustrated in FIG. 1. Although these defects can be reduced to underfifty counts by gently wiping the target, after another several kwhsputter deposition, the surface of the target will again exhibit severedefect formation. These target defects in turn cause increasing defectformation in and on the deposited film. FIGS. 2( a) and 2(b) representtypical scanning electron microscope (SEM) images of defects formed ondeposited silver selenide films.

Based on the film deposition rate for silver selenide, targetconsumption of about 2-3 kwh represents deposition of silver selenide ononly several hundred wafers. This wafer deposition rate is not practicalfor commercial production. Furthermore, in order to obtain the properfilm composition and resistivity, the silver selenide target requires atleast 0.5 kwh of conditioning each time the target chamber is vented fortarget cleaning. Thus, the usable target life between cleanings is evenfurther reduced.

The defect shapes shown in FIGS. 2( a) and (b) lead the presentinventors to believe that the deposited silver selenide defects could bedue to a micro arching on the silver selenide target, which subsequentlyforms a splatter defect in and on the silver-selenide film. This beliefis supported by FIGS. 3( a) and (b), showing two images of an Ag₂Setarget after 3 kwh of target consumption. The images clearly show manyblack porous chips, which are approximately millimeter sized. FIGS. 3(a) and (b) also show that the defect density reaches its highest on theracetrack portion of the target.

Silver selenide (e.g. Ag₂Se) is well known for its low temperature phasetransition point of 406 K (about 130° C.). At temperatures below 406 K,Ag₂Se forms an orthorhombic structure, known as the “beta phase.” Attemperatures above 406 K (about 133° C.), Ag₂Se undergoes a structuralchange in which the Se forms a body-centered cubic sublattice, while theAg undergoes a melting transition. In this so-called “alpha phase” or“superionic phase,” the Ag ions exhibit liquid-like diffusion. At around1170 K (897° C.), the Se sublattice will also undergo a meltingtransition.

During magnetron sputter deposition, the high electrical potential andcurrent applied to the target as well as the energy imparted to thetarget via bombardment by the inert gas ions can cause localized heatingof the sputter target in the regions where the high-density plasmadischarge is concentrated. This heating effect is sufficient to raisethe temperature of localized regions of the Ag₂Se target at least enoughto accelerate the diffusion of Ag through the Ag₂Se target. Thisdiffusion can cause natural protrusion growth on the target, which inturn explains the origin of the porous chip defects formed on the Ag₂Setarget.

The origin of the black porous chip defects is further explained withreference to FIGS. 4( a), (b), (c), (d), (e), and (f), which show aseries of SEM images taken on silicon oxide wafers on which Ag₂Se filmswere sputter deposited and then in situ annealed at varyingtemperatures. In this experiment, wafer stages were brought up tovarious temperatures at least thirty minutes prior to loading a waferonto the stage. Wafers were then loaded onto the wafer stages andannealed for 5 minutes. Each wafer was then loaded into a coolingchamber and cooled back to room temperature. All deposition, annealing,and cooling processes were in situ processes, which reduces the risk ofcontamination problems.

FIG. 4( a) shows a silicon wafer with a deposited silver selenide filmat room temperature. The as-deposited film appears smooth. FIG. 4( b)shows a silicon wafer with a deposited silver selenide film afterannealing at 130° C. As illustrated in FIG. 4( b), the silver selenidefilm clearly shows a structure change. FIG. 4( c) shows a silicon waferwith a deposited silver selenide film after annealing at 250° C., whileFIG. 4( d) shows a silicon wafer with a deposited silver selenide filmafter annealing at 350° C. The silver-selenide film of FIG. 4( d)clearly shows the development of a cone-like defect. FIGS. 4( e) and4(f) show silicon wafers with deposited silver selenide films afterannealing at 380° C. and 410° C. respectively. The silver selenide filmsof FIGS. 4( e) and 4(f) show a progression in defect size from the conedefect of FIG. 4( d) to larger whisker-like defects. These whisker-likedefects are on the millimeter scale in size and are clearly visible onthe wafer with the unaided eye. Accordingly, comparison of the defectformation on the annealed silver selenide films with the defectformation observed on the Ag₂Se target during sputtering lead thepresent inventors to understand that controlling the target temperatureduring Ag₂Se deposition is desirable for controlling defect formation inand on the target and the deposited film.

FIG. 11 shows a simplified example of a magnetron sputtering device 100,similar to that used in the exemplary embodiments described below.Generally, the device 100 comprises a substrate support table 102,target 104, target backing plate 105, and a magnetron 108. Duringsputtering, a substrate 101 is placed on the substrate support table 102and raised to a position near the target 104 inside the chamber 116.Using a pump 122, air in the sputtering chamber is evacuated andreplaced with an inert sputter gas, usually at a low pressure. Thesputter gas may be any suitable sputtering gas such as argon, neon,xenon and combinations thereof. A motor assembly 112 provides rotationalmotion to the magnetron 108 through a shaft 114 to rotate the magnetron108 at about 100 rpm. The plasma discharge occurs in the space betweenthe substrate and the target 104, coating the substrate with sputteredtarget particles. As described above, this process may cause heating ofthe target, which is disadvantageous to silver selenide deposition.

An exemplary system of the magnetron sputtering device 100 describedabove is the Applied Materials Endura magnetron sputtering system usingan ENI pulsed DC power supply. Accordingly, except where indicated,silver selenide deposition according to the exemplary embodiments of thepresent invention discussed below was performed using an AppliedMaterials Endura magnetron sputtering system using an ENI pulsed DCpower supply with a 350 W, 200 KHz, 1056 ns pulse width. The sputteringtarget used in each exemplary embodiment was a silver selenide targetapproximately 13 inches in diameter and approximately ⅛ inch thick. Thesilver selenide target purity was at least 99.95%. All embodiments ofthe present invention are designed to maintain a silver selenide targettemperature at less than about 350° C. and preferably less than about250° C. during sputtering.

As discussed above, silver selenide undergoes a structural change fromthe “beta phase” to the “alpha phase,” in which the Se forms abody-centered cubic sublattice, at temperatures above 406 K (about 133°C.). In accordance with the present invention, it is desirable for thedeposited silver selenide film to be formed from a mixture of the alphaand beta silver selenide phases. Such mixture is critical for theoptimal performance of certain devices formed from silver selenidefilms. Conventional silver selenide sputter techniques produce depositedsilver selenide films predominantly consisting of beta silver selenide.According to the present invention, silver selenide films containingboth alpha and beta phases may be formed via a sputtering process underconditions of lower power/higher pressure. Furthermore, formation ofalpha phase silver selenide is enhanced by heating of the substrate.

FIG. 13 shows two x-ray diffraction (XRD) scans 300, 320 of sputterdeposited silver selenide films. Scan 1 shows an XRD scan 300 of asilver selenide film sputter deposited at a sputter power of 350 W andan argon sputter gas flow rate of 50 sccm, which corresponds to asputter gas pressure of about 7 mTorr. Scan 2 shows an XRD scan 320 of asilver selenide film sputter deposited at a sputter power of 350 W andan argon sputter gas flow rate of 75 sccm, which corresponds to asputter gas pressure of about 10 mTorr. The scans 300, 320 show that,under the latter conditions, alpha and beta peaks appear around 23 and37 degrees in a glancing mode 2θ XRD scan.

While not being limited by theory, based on the XRD scans 300, 320, itappears that a lower energy impact of silver selenide with the substrateduring sputtering yields more alpha phase silver selenide in thedeposited film. Lower sputter power and higher sputter pressure decreasethe kinetic energy imparted to the sputtered silver selenide molecules,thus resulting in a lower energy impact of silver selenide with thesubstrate. Furthermore, it also appears that heating the substrate to ana temperature above room temperature (about 25 C) also enhances thealpha phase in the deposited film.

While the crystalline structure of silver selenide (i.e. mixture ofalpha and beta phases) is important for the optimal performance ofdevices employing such films, the film texture (e.g. smoothness), anddefects in or on the film also affect device performance. Thus, to theextent decreases in sputter power or increases in sputter pressureresult in films exhibiting a rough or defect-laden surface, a balance inthe sputter power and pressure parameters must be struck in order toproduce smooth, defect-free silver selenide films, whose structureexhibits both alpha and beta phases. However, it should be emphasizedthat, independent of the process method used to produce the film, it isdesirable to have the alpha phase present in deposited silver selenidefilms.

In a sputtering process in accordance with a first exemplary embodimentof the invention, a silver selenide film is sputter deposited using themagnetron sputtering system described above, but maintaining a sputterpower such that the temperature of the silver selenide target 104 ismaintained at less than about 350° C. and preferably less than about250° C. during sputtering.

Generally, reducing the sputter power reduces the kinetic energyimparted to the target during sputtering and, thus, lowers targettemperature. As shown in FIG. 5, target temperature is directly relatedto sputter power such that increasing sputter power increases targettemperature. For the sputtering system 100 described above and asillustrated in FIG. 5, target temperature is maintained at less than350° C. at sputter powers below about 200 W. As also shown in FIG. 5,target temperature can be further reduced to below about 250° C. atsputter powers less than about 100 W.

In accordance with a second exemplary embodiment of the invention, asilver selenide film is deposited using the Applied Materials Enduramagnetron sputtering system described above, but maintaining a sputtergas pressure less than about 40 mTorr and preferably less than about 10mTorr so as to maintain a temperature of the silver selenide target ofless than about 350° C. and preferably less than about 250° C. duringsputtering.

As indicated in FIG. 6, increasing the sputter gas pressure lowers thetarget temperature during sputtering. This reduction in temperature assputter gas pressure increases is due to a reduction in the kineticenergy of bombardment imparted to the target by the impact of the inertgas ions during sputtering. FIG. 6 illustrates the correlation betweentarget temperature and sputter pressure for the sputter processdescribed above, using argon as the sputter gas. According to FIG. 6,silver selenide target temperature is reduced to less than about 350° C.at a sputter pressures greater than about 5 mTorr. Furthermore, targettemperature is reduced to less than about 250° C. at sputter pressuresgreater than about 15 mTorr.

Referring now to FIGS. 7-9 and in accordance with a third exemplaryembodiment of the invention, a silver selenide film is sputter depositedusing the Applied Materials Endura magnetron sputtering system describedabove, but using a sputter gas with a molecular weight at least greaterthan that of neon such that the silver selenide target temperature ismaintained at about 350° C. and preferably less than about 250° C.during sputtering. While the preferred sputtering gas is xenon, thepresent invention is not so limited. The inventors have discovered thatat least xenon, argon, and mixtures of these induce the desiredreduction in silver selenide target temperature as shown in FIGS. 8 and9.

Sputter gasses with higher molecular weight than that of neon, such asargon and xenon, reduce the kinetic energy imparted to the target duringsputtering. FIG. 7 shows the correlation between the sputter targettemperature and Ne/Ar sputter gas flow ratio at various sputter gaspressures. According to FIG. 7, at least at the lower sputter pressuresindicated, increasing the proportion of neon in the sputter gas mixinduces higher sputter target temperatures. Furthermore, when the Ne gasin the Ne/Ar sputter gas flow ratio of FIG. 7 is replaced with xenongas, as shown in FIG. 8, there is a clear decrease in target temperatureat the sputter gas pressures indicated. FIG. 9 also shows therelationship between target temperature and sputter gas at varioussputter gas pressures. As shown in FIG. 9, using a sputter gascontaining a mixture of argon and neon results in higher targettemperatures than use of either argon or xenon alone.

FIG. 8 also shows that, as the molecular weight of the sputter gasincreases (i.e. increasing Xe/Ar ratio) target temperature is quicklyreduced to at least less than about 350° C. at a sputter pressure of 2mTorr. At the relatively higher pressures shown in FIG. 8 (10 mTorr and20 mTorr), the target was cooled to less than about 250° C. Sputter gaspressures of less than about 10 mTorr are desirable because they havebeen shown to prevent the formation of nodular defects in the depositedsilver selenide films with high silver concentrations (e.g. higher thanabout 67.5% Ag). Furthermore, lower sputter gas pressures allow for thedeposition of silver selenide films with greater stoichiometricprecision.

In accordance with a fourth exemplary embodiment of the invention,silver selenide target cooling is achieved by improving the targetcooling apparatus of the sputter deposition system. Referring to FIG.11, a typical magnetron sputtering device 100, described above,comprises a magnetron 108, which is disposed within a cooling chamber116. The cooling chamber 116 is defined by a top 117, two sides 119 andthe target backing plate 105. A cooling fluid, such as water, enters thecooling chamber 116 at an inlet 118, circulates around the magnetron108, and exits through an outlet 120. The arrows A-F show thegeneralized water flow paths around the magnetron 108.

The present inventors have discovered that silver selenide targetcooling to at least below 350° C. and preferably less than about 250° C.can be achieved by increasing the coolant flow between the magnetron 108and the target backing plate 105 (as shown by arrows C and D),decreasing coolant temperature, and improving the thermal conductivitybetween the coolant and target backing plate 105. For example, coolantflow rates are at least above 2.5 gal/min and coolant temperature is atleast below 25° C. to allow selenide target cooling to at least below350° C. and preferably less than about 250° C. Furthermore, the backingplate should be constructed from materials designed to optimize backingplate/coolant contact and the target backing plate should be coloredblack in order to improve heat radiation.

In accordance with a fifth exemplary embodiment of the invention, silverselenide target temperature is maintained below about 350° C. andpreferably less than about 250° C. during sputtering by maintaining aneffective sputter power density on the target of less than about 1W/cm².

The target sputter area of a sputtering apparatus will vary according tothe particular apparatus used. As described below, the area of thetarget actually used during sputtering can range from only a portion ofthe entire target, as shown in FIGS. 12( a) and 12(b), to the entiretarget, as shown in FIG. 12( c). According to the present invention,sputter parameters, such as the sputter power, for a particular sputterapparatus are varied such that the effective sputter power density ofthe sputter apparatus is less than about 1 W/cm².

As described above, FIG. 5 shows a graph of Target Temperature v.Sputter Power for the Applied Materials Endura Sputter system describedabove, using argon as the sputter gas, and clearly indicates an increasein target temperature as sputter power is increased. However, thesputter area of the target, also impacts target temperature. Thus,controlling effective sputter power density (sputter power per unit ofsputter area) provides more accurate control of silver selenide targettemperature. Silver selenide target temperature as a function ofeffective sputter power density is plotted in FIG. 10 and clearly showsthat silver selenide target temperature is maintained below about 350°C. and preferably less than about 250° C. during sputtering bymaintaining an effective sputter power density on the target of lessthan about 1 W/cm².

FIG. 12( a) shows a cross sectional view of a non-uniform target sputterprofile, which represents the sputter area of the target. Duringsputtering, areas of target erosion, S₁ and S₂, are caused by thehigh-density plasma as it moves across the target within the magneticfiled generated by the magnetron. As described above, thispreferentially sputtered region of the target is often referred to asthe racetrack portion, due to the characteristic ovoid-shaped area ofthe target that is eroded by the high-density plasma discharge. FIG. 12(b) illustrates a typical racetrack area 200 formed on a target 104during sputtering. As shown, the total sputter area S_(total) of thetarget is determined by the formula:S _(total)=π(R ₂ ² −R ₁ ²)Effective sputter power density on the target can then be determined bydividing the sputter power of the sputtering system by S_(total).

FIG. 12( c) shows a cross sectional view of a uniform target sputterprofile. In this case, the sputter power is spread across the entiretarget area S_(total). Similar to the non-uniform target sputterprofile, sputter power density on the target can be determined bydividing the total sputter power by S_(total).

In accordance with a sixth exemplary embodiment of the presentinvention, the magnetron in a magnetron sputtering system is positionedsuch that the target temperature is maintained below about 350° C. andpreferably less than about 250° C. during sputtering.

As described above, FIG. 11 shows a typical magnetron sputtering device100 comprising a target 104, target backing plate 105, and a magnetron108. As the distance between the magnetron and the sputter target 107 isincreased, the distribution of the magnetic field on the target alsoincreases, causing the sputter area of the target to increase.Increasing sputter area has the effect of decreasing sputter powerdensity, which, as described above, lowers target temperature duringsputtering.

In accordance with a seventh exemplary embodiment of the presentinvention, a silver selenide film is sputter deposited under sputterconditions such that the structure of said silver selenide filmcomprises both the alpha phase and beta phases. Desirable sputterdeposited silver selenide films consist primarily of alpha phase silverselenide. For example, using the applied Materials Endura magnetronsputtering system described above, silver selenide is sputter depositedusing a sputter power of less than about 250 W and a sputter pressure ofat least about 10 mTorr. Furthermore, under sputter conditions whereinthe sputter power is less than about 200 W and the sputter pressure isabout 10 mTorr or less, a defect-free silver selenide film comprisingboth the alpha and beta structural phases may be formed on a substrate.

Regardless of the sputtering apparatus used, heating the substrate togreater than about 30° C. results in greater formation of alpha phasesilver selenide in the deposited film.

The present invention uses novel methods and apparatuses for silverselenide sputter deposition that decrease defects in deposited silverselenide films. Such methods and apparatuses of silver selenide defectcontrol are useful in the production of certain non-volatile memorydevices that require virtually defect-free silver selenide film layers.Furthermore, the present invention provides novel methods andapparatuses for silver selenide sputter deposition that are suitable toindustrial application.

While separate exemplary embodiments of the invention have beendescribed and illustrated, practice of the present invention is notlimited to use of only one of these exemplary embodiments. One or moreof the embodiments of the invention can be used separately or togetherto maintain a sputter target at a temperature of less than about 350° C.and preferably less than about 250° C.

Furthermore, while exemplary embodiments of the invention have beendescribed and illustrated, various changes and modifications may be madewithout departing from the spirit or scope of the invention.Accordingly, the invention is not limited by the foregoing description,but is only limited by the scope of the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method comprising forming a memory device,the forming of the memory device comprising: providing a substrate; andsputter depositing a silver selenide film on a substrate to form asilver selenide film which comprises both alpha silver selenide and betasilver selenide.
 2. The method of claim 1, wherein the sputteringcomprises providing a sliver selenide target and maintaining the silverselenide target at a temperature of less than about 350° C. during thesputtering process.
 3. The method of claim 2, wherein maintaining thetarget temperature of less than about 350° C. is achieved by injecting asputter gas having a molecular weight greater than a molecular weight ofneon into a sputter deposition chamber.
 4. The method of claim 3,wherein the sputter gas is argon, xenon, or a combination of argon andxenon.
 5. The method of claim 3, wherein the sputter gas is acombination of argon and xenon, wherein the flow ratio of the xenon toargon is equal to or greater than about 0.2 and wherein the sputterpressure is equal to or greater than about 2 mTorr.
 6. The method ofclaim 2, wherein maintaining the silver selenide target temperature ofless than about 350° C. is achieved by maintaining a sputter power ofless than about 200 W during sputtering.
 7. The method of claim 2,wherein maintaining the silver selenide target temperature of less thanabout 350° C. is achieved by maintaining an effective sputter powerdensity of less than about 1 W/cm².
 8. The method of claim 2, whereinmaintaining the silver selenide target temperature of less than about350° C. is achieved by maintaining a sputter gas pressure of less thanabout 40 mTorr.
 9. The method of claim 2, wherein maintaining the silverselenide target temperature of less than about 350° C. is achieved bymaintaining a sputter gas pressure of less than about 10 mTorr.
 10. Themethod of claim 2, wherein maintaining the silver selenide targettemperature of less than about 350° C. is achieved by positioning amagnetron a distance from the target so as to maintain a targettemperature of less than about 350° C.
 11. The method of claim 2,wherein maintaining the silver selenide target temperature of less thanabout 350° C. is performed by cooling the silver selenide sputter targetwith a cooling apparatus.
 12. The method of claim 2, wherein the silverselenide target temperature is maintained at less than about 250° C.during the sputtering process.
 13. A method comprising: forming a memorydevice, the forming of the memory device comprising: providing asubstrate and a silver selenide sputter target in a sputter depositionchamber; and sputter depositing a silver selenide film on a substrate toform a silver selenide film which comprises both alpha silver selenideand beta silver selenide by injecting a sputter gas having a molecularweight greater than a molecular weight of neon into the sputterdeposition chamber.
 14. The method of claim 13, wherein the sputter gasis argon, xenon, or a combination of argon and xenon.
 15. The method ofclaim 13, wherein the sputter gas is a combination of argon and xenon,wherein the flow ratio of the xenon to argon is equal to or greater thanabout 0.2 and wherein the sputter pressure is equal to or greater thanabout 2 mTorr.